(1). Field of the Invention
The present invention relates to a method for dividing a substrate that is applicable to a method for manufacturing, for example, a semiconductor laser element, a light-emitting diode, and a field-effect transistor integrated circuit that are made up of nitride semiconductors.
(2). Description of the Related Art
A GaN-based nitride semiconductor (InGaAIN), which is represented by the composition formula AlxGa1-x-yInyN (0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦x+y+z≦1) and has a wide band gap (the band gap width of GaN at room temperature is 3.4 eV), is a material that can realize a high-power light-emitting diode in a wavelength range such as the green/blue visible range and the ultraviolet region. A widely used light-emitting diode realized by InGaAIN is a white light-emitting diode that obtains white light by exciting a fluorescent substance by a blue light-emitting diode. Furthermore, a violet semiconductor laser element using a nitride semiconductor is already in commercial use as a next-generation high-density optical disc light source. Moreover, since a nitride semiconductor has the characteristics of a high saturated drift velocity and a high breakdown voltage, it is considered as a promising material for a high-frequency and high-power electronic device in the future, and research and development on it has been actively conducted.
In general, a method is employed that uses a highly rigid substrate such as a sapphire substrate and a SiC substrate for crystal growth and that epitaxially grows a semiconductor layer on such substrate using the metal organic chemical vapor deposition (DOCVD). Moreover, recent years have seen the development of a GaN substrate that is obtained by forming a thick-film crystal on a base material substrate by the hydride vapor phase epitaxy (HVPE) and then by separating or removing the base material substrate. Furthermore, formation of a device structure on a GaN substrate has also been carried out in recent years. In either cases, it is generally very difficult to perform chip separation since these substrates are highly rigid compared with conventional semiconductor substrates such as a Si substrate and a GaAs substrate. Thus, a generally used method that performs dicing with a diamond blade (e.g., see the Japanese Laid-Open Patent application No. 08-236867 and the Japanese Laid-Open Patent application No. 10-242570) has problems such as frequent occurrence of chip breaking and the difficulty of dicing each substrate into chips in the same square-like form. Meanwhile, in the case of manufacturing a semiconductor laser element, it is necessary to form a resonant mirror by means of cleavage, but it is difficult to make the resulting cleaved facet flat. Conventionally, cleavage or the like has been performed by forming a linear ditch on a sapphire substrate or a SiC substrate using a diamond scriber and then by pressing an edge jig against the substrate. However, since it is difficult by this method to obtain a flat cleaved facet, there is a problem that the threshold current of the resulting semiconductor laser element becomes higher and the process yield becomes worse.
Another example of the case where dicing is difficult is chip separation of a substrate on which a Si large scale integrated circuit (LSI) with a highly rigid, low dielectric insulating film is formed. Development on a Si LSI has been conducted in an accelerated rate for reducing its size down to the deep submicron region as well as for increasing the operation speed of such down-sized Si LSI, but wiring delay is a significant problem for such an LSI. As a solution to this problem, an active attempt has been made to reduce wring delay by reducing the dielectric constant of an insulating film between wires. However, since the material of this low dielectric insulating film is generally very rigid, there is a problem that it is very difficult to perform dicing using a diamond blade for chip separation.
As described above, it is very difficult to divide each semiconductor substrate into chips in the same desired form without causing chip breaking in the case where such semiconductor substrate is a nitride semiconductor substrate that is made of a nitride semiconductor device formed on a sapphire substrate or a SiC substrate or where it is a semiconductor substrate that is made of a semiconductor device, including a highly rigid material such as a low dielectric insulating film, formed on a sapphire substrate or a Sic Substrate. Thus, there is a call for technology for semiconductor substrate cleavage and chip separation that is capable of solving the above problems.
The following describes two conventional methods for dividing a semiconductor substrate as examples of the conventional arts.
FIG. 1A and FIG. 1B are an external view and a cross-sectional view, respectively, showing a conventional method for cleaving a nitride semiconductor substrate.
First, as shown in FIG. 1A, a GaN-based semiconductor laser element is formed by forming an epitaxial growth layer 13 on a sapphire substrate 7 using, for example, the MOCVD method. More specifically, such epitaxial growth layer 13 includes an n-type AlGaN cladding layer, an InGaN multi-quantum well active layer, and a p-type AlGaN cladding layer. The InGaN multi-quantum well active layer generates a violet laser oscillation at 405 nm. The p-type AlGaN cladding layer or the p-type GaN layer is formed as the surface of the epitaxial growth layer 13, and a patterned p-type ohmic electrode such as Ni/Au is formed on the p-type AlGaN cladding layer. An n-type ohmic electrode such as Ni/Al is formed on the n-type AlGaN cladding layer that has been exposed at the surface after selectively removing the p-type AlGaN cladding layer or the InGaN multi-quantum well active layer or on the n-type GaN layer that is formed below such n-type AlGaN cladding layer. Note that the sapphire substrate is taken as an example here, but the substrate may be a SiC substrate. Subsequently, the rear surface of the sapphire substrate 7 on which the epitaxial growth layer 13 is not formed is polished until the thickness of the sapphire substrate 7 becomes, for example, about 100 μm, and then scribe lines 15 are formed on the rear surface of the sapphire substrate 7 at intervals of the resonant length of the semiconductor laser element in the a axis direction (<11-20>direction) that is the direction in which the sapphire substrate 7 is to be cleaved. A diamond scriber 14 is used to form these scribe lines 15, each being a ditch with a depth of about 50 μm.
After the formation of the scribe lines 15, as shown in FIG. 1B, a bar-shaped nitride semiconductor substrate made up of plural semiconductor laser chips is formed by placing an edge jig 17 on each of the scribe lines 17 formed on the rear surface of the sapphire substrate 7 and then by applying pressure from the front surface of the epitaxial growth layer 13 using a jig 16. Then, semiconductor laser chips are obtained by repeatedly performing: the application of coating to the resulting cleaved facet 18 of the bar-shaped nitride semiconductor substrate for a better edge face reflectivity; and the above cleavage process to further divide the bar-shaped nitride semiconductor substrate.
FIG. 2 is an external view of a conventional chip separation method for semiconductor substrate.
First, as shown in FIG. 2, a GaN-based epitaxial growth layer 19 is formed on a sapphire layer 7 using, for example, the MOCVD method. This epitaxial growth layer 19 constitutes a light-emitting diode or a field-effect transistor integrated circuit. More specifically, in the case of constituting a light-emitting diode, it includes: an n-type GaN layer or an n-type AlGaN layer; an InGaN multi-quantum well active layer; and a p-type AlGaN layer or a p-type GaN layer. The InGaN multi-quantum well active layer emits a blue light of 470 nm by current injection. Meanwhile, in the case of constituting a field-effect transistor, an n-type AlGaN layer is formed on an undoped GaN layer. Subsequently, the sapphire substrate 7 is made into a thin-film by means of polishing or the like after the completion of the device formation process such as the formation of an electrode. After this, as shown in FIG. 2, it is possible to perform chip separation by cutting the semiconductor substrate into squares in the x and y directions using a diamond blade 20.